Prior art concerning solar cells known to one knowledgeable in the art is present in the literature. References contained in U.S. Pat. No. 4,128,733, U.S. Pat. No. 5,686,734, U.S. Pat. No. 6,333,457, U.S. Pat. No. 6,423,568, U.S. Pat. No. 6,743,974, U.S. Pat. No. 6,745,687, U.S. Pat. No. 6,998,288, U.S. Pat. No. 7,030,313, U.S. Pat. No. 7,179,677, U.S.2002/0040727, U.S.2004/0200520, U.S.2005/0000566 are cited as prior art and included herein in their entirety by reference. Recent silicon (Si) solar cell devices are manufactured using generally bulk and or thin film configurations. Typically, bulk Si solar cells are classed as first generation devices. In an effort to reduce cell cost, the volume of Si required is reduced using thin films of Si on relatively cheaper substrates, such as glass (e.g., SiO2). Thin film semiconductor solar cell approaches form second generation devices, generally. Unfortunately, depositing high quality single crystal (or monocrystalline) Si on amorphous substrates has proved extremely difficult. Typically, the Si deposited on glass substrates is amorphous. Efforts to produce amorphous Si (a-Si) solar cells have consistently shown inferior performance compared to single crystal bulk Si solar cells. To improve the crystal quality of the a-Si films, they must be heat treated to temperatures approaching the melting point of Si (Tmelt˜1420° C.) in order for recrystallization to occur. The result of which is either polycrystalline (poly-Si) and or large domain single crystal Si. Again, the poly-Si and/or large domain single crystal Si (ld-sc-Si) thin film solar cells have energy conversion efficiency below single crystal bulk Si solar cells. Both first and second generation Si solar devices are based on a single junction (SJ) configuration. A limitation of SJ's is only a small optical energy absorption window in the immediate vicinity of the fundamental energy band gap can be used advantageously, thereby rejecting a large portion of the available power from the solar spectrum. It has been (theoretically) shown by workers in the field that the maximum attainable energy conversion efficiency for SJ cells is η(SJ)≦25-32%. The present invention solves a long standing problem of detrimental high energy photon effects in Si solar cell devices.
The superior crystal quality of bulk Si substrates manufactured using Czochralski (CZ) growth techniques is due to requirements for silicon based ultra-large-scale-integrated-circuits (ULSICs) based on complementary-metal-oxide-semiconductor (CMOS) transistors. Single crystal silicon (sc-Si) substrates with diameters of 300 mm are presently in widespread CMOS production with plans to implement 450 mm in the near future. A unique aspect of ULSI CMOS industry is the extremely successful manipulation of large form factor substrates using area fabrication tools, such as, ion implantation, thin film deposition and lithography. This allows high complexity planar structures to be economically manufactured with high throughput—i.e., wafer scale manufacture.
The silicon solar cell industry in comparison can be well described as a discrete fabrication technology with extremely low levels of integration. For example, a single junction Si solar cell typically delivers less than 0.7V and large numbers of discrete cells must be interconnected into modules in order to generate useful voltages and current for power generation. Furthermore, each cell must be separately packaged and environmentally sealed. The present invention discloses wafer scale manufacture of SJ silicon modules using high throughput and large area substrates. Furthermore, the present invention discloses large area thin film Si transfer technique onto cost effective substrates. The device fabrication methods disclosed allow complex power systems with low cost when applied to high volume throughput. As a general observation, a solar power fabrication plant producing 1 gigawatt per year using silicon SJ devices will consume approximately 150-200 times more Si substrate area than a 300 mm CMOS plant.
Solar Energy Conversion Devices
The broadband solar optical spectrum at ground level ranges from below 300 nm to over 1700 nm, spanning the ultraviolet to far infrared. FIGS. 1A and B show a general power spectrum, punctuated with multiple atmospheric absorption regions. The peak spectral variance is seen to occur in the 400-600 nm region. Optical photon to electron conversion devices employing semiconductors are well known. FIG. 2 shows the absorption coefficient αabs of several technologically mature semiconductors. The indirect bandgap semiconductors Si and germanium (Ge) span major portions of the solar spectrum. Group III-V direct band gap compound semiconductors, such as, gallium arsenide (GaAs), indium phosphide (InP) and indium gallium arsenide (InGaAs) alloy also have good spectrum coverage and exhibit sharp cut-on wavelengths defined by the direct band edges. Wide band gap silicon-carbide (6H:SiC) absorbs only at the highest energies in the UV, and covers the least fraction of the solar spectrum. In comparison Si and Ge, have long wavelength absorption tails due to the indirect energy-momentum band structure; see FIGS. 3A-C. If an optical photon incident upon the Si crystal has energy equal to or above the fundamental band gap energy it is absorbed. This creates an electron-hole pair with the aid of an appropriate lattice phonon wavevector-k which is required in the photocarrier generation process in order to conserve energy and momentum. The inverse process of electron-hole (e-h) recombination is extremely inefficient compared to direct band gap semiconductors. For the present case of optical to electronic conversion, indirect band gap Si is advantageous for high sensitivity photodetection compared to direct band gap semiconductors—where e-h radiative recombination is efficient and represents a significant loss mechanism. Looking closely at the absorption coefficient of Si in FIG. 2 and FIGS. 3A-C, it is clear that the absorption depth near the Si fundamental band gap (Eg˜1.1 eV) is extremely long. This means that photons with energy equal to or slightly greater than band gap energy Eg(Si) will penetrate to a depth Le=1/αabs, deep within the crystal. However, the highest absorption coefficient for all the semiconductors is found for Si for wavelengths shorter than ˜400 nm. Silicon photodetectors (SiPDs) have been shown to exhibit very low noise, high sensitivity and efficient avalanche multiplication effects. The low noise property is due to the small probability of radiative recombination due to the intrinsic indirect energy bandgap structure. The spectral range of SiPDs spans the broad range 200 nm to 1200 nm, and will be discussed later for application to UV solar cell conversion.